In today's radio communications networks a number of different technologies are used, such as Long Term Evolution (LTE), LTE-Advanced, Wideband Code Division Multiple Access (WCDMA), Global System for Mobile communications/Enhanced Data rate for GSM Evolution (GSM/EDGE), Worldwide Interoperability for Microwave Access (WiMax), or Ultra Mobile Broadband (UMB), just to mention a few possible implementations. A radio communications network comprises radio network nodes such as radio base stations providing radio coverage over at least one respective geographical area forming a cell. The cell definition may also incorporate frequency bands used for transmissions, which means that two different cells may cover the same geographical area but using different frequency bands. User equipments (UE) are served in the cells by the respective radio network node and are communicating with respective radio network node. The user equipments transmit data over an air or radio interface to the radio network node in uplink (UL) transmissions and the radio network nodes transmit data over an air or radio interface to the user equipments in downlink (DL) transmissions.
In for example 3rd Generation Partnership Project (3GPP) LTE, transmission gain is increased using transmission diversity and beamforming transmission. The benefits of beamforming are to increase the received signal gain, by making signals emitted from different antennas add up constructively, and to reduce the multipath fading effect. Adaptive transmit beamforming in the radio communications network aims at maximizing the power received by the intended user equipment while at the same time minimizing the interference transmitted to other user equipments. Closed form solutions for the optimal transmit antenna weights may in many cases be found if one puts a constraint on the total transmitted power at the radio network node in the optimization, e.g. zero-forcing beamforming. For zero-forcing beamforming in downlink, an algorithm allows the radio network node to send data to the desired user equipments together with nulling out a direction to undesired user equipments; and for uplink, the radio network node receives from the desired user equipments together with nulling out the directions from the interference user equipments.
A more relevant constraint for practical radio communications networks is to set a limit on the maximum power that each Power Amplifier (PA) may deliver. However, this leads to a more complicated optimization problem which typically does not have a closed form solution. Therefore, one has to resort to numerical optimization which may render a real-time application of the method impractical.
A problem with existing solutions is that the optimal weights do not have equal amplitude which will lead to poor utilization of the available PA resources. In most radio network node architectures the relevant constraint is on the maximum power transmitted from one radio branch, rather than on the total transmitted power. This means that if the transmit weights do not have equal amplitude, some of the radio branches will not transmit with full power. The PAs in these branches will then also work at a load where they have poor efficiency. This results in a reduced performance of the communication within the radio communications network.